U.S. patent number 9,783,876 [Application Number 14/387,261] was granted by the patent office on 2017-10-10 for stainless steel for oil wells and stainless steel pipe for oil wells.
This patent grant is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The grantee listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hisashi Amaya, Kunio Kondo, Shinjiro Nakatsuka, Taro Ohe, Tomohiko Omura, Yohei Otome, Masanao Seo, Hideki Takabe, Yusaku Tomio.
United States Patent |
9,783,876 |
Nakatsuka , et al. |
October 10, 2017 |
**Please see images for:
( Certificate of Correction ) ** |
Stainless steel for oil wells and stainless steel pipe for oil
wells
Abstract
A stainless steel for oil wells which has excellent
high-temperature corrosion resistance and can stably obtain a
strength of not less than 758 MPa is provided. The stainless steel
for oil wells contains, by masse, C: not more than 0.05%, Si: not
more than 1.0%, Mn: 0.01 to 1.0%, P: not more than 0.05%, S: less
than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0
to 5.5%, Co: 0.01 to 1.0%, Al: 0.001 to 0.1%, O: not more than
0.05%, and N: not more than 0.05%, the balance being Fe and
impurities, and satisfies Formulas (1) and (2):
Cr+4Ni+3Mo+2Cu.gtoreq.44 (1) Cr+3Ni+4Mo+2Cu/3.ltoreq.46 (2) where
each symbol of element in Formulas (1) and (2) is substituted by
the content (mass %) of a corresponding element.
Inventors: |
Nakatsuka; Shinjiro (Tokyo,
JP), Ohe; Taro (Tokyo, JP), Amaya;
Hisashi (Tokyo, JP), Takabe; Hideki (Tokyo,
JP), Otome; Yohei (Tokyo, JP), Tomio;
Yusaku (Tokyo, JP), Seo; Masanao (Tokyo,
JP), Omura; Tomohiko (Tokyo, JP), Kondo;
Kunio (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION (Tokyo, JP)
|
Family
ID: |
49259321 |
Appl.
No.: |
14/387,261 |
Filed: |
February 27, 2013 |
PCT
Filed: |
February 27, 2013 |
PCT No.: |
PCT/JP2013/055219 |
371(c)(1),(2),(4) Date: |
September 23, 2014 |
PCT
Pub. No.: |
WO2013/146046 |
PCT
Pub. Date: |
October 03, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150047831 A1 |
Feb 19, 2015 |
|
Foreign Application Priority Data
|
|
|
|
|
Mar 26, 2012 [JP] |
|
|
2012-068598 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
38/50 (20130101); C22C 38/06 (20130101); C22C
38/44 (20130101); C21D 6/004 (20130101); C21D
9/08 (20130101); C22C 38/002 (20130101); C22C
38/54 (20130101); C22C 38/46 (20130101); C22C
38/004 (20130101); E21B 17/00 (20130101); C22C
38/52 (20130101); C22C 38/04 (20130101); C22C
38/02 (20130101); C22C 38/48 (20130101); C22C
38/001 (20130101); C21D 6/002 (20130101); C22C
38/42 (20130101); C22C 38/005 (20130101); C21D
2211/008 (20130101); C21D 2211/005 (20130101) |
Current International
Class: |
C22C
38/42 (20060101); C22C 38/46 (20060101); C22C
38/06 (20060101); C22C 38/02 (20060101); C21D
9/08 (20060101); C22C 38/00 (20060101); C22C
38/54 (20060101); C22C 38/48 (20060101); C22C
38/50 (20060101); C22C 38/52 (20060101); C22C
38/44 (20060101); E21B 17/00 (20060101); C21D
6/00 (20060101); C22C 38/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2123470 |
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Nov 1994 |
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CA |
|
2762899 |
|
Dec 2010 |
|
CA |
|
1571861 |
|
Jan 2005 |
|
CN |
|
2002-004009 |
|
Jan 2002 |
|
JP |
|
2005-336595 |
|
Dec 2005 |
|
JP |
|
2006-016637 |
|
Jan 2006 |
|
JP |
|
2007-009321 |
|
Jan 2007 |
|
JP |
|
2007-277639 |
|
Oct 2007 |
|
JP |
|
2007-332442 |
|
Dec 2007 |
|
JP |
|
2008-050646 |
|
Mar 2008 |
|
JP |
|
2009/119048 |
|
Oct 2009 |
|
WO |
|
2010/050519 |
|
May 2010 |
|
WO |
|
2010/134498 |
|
Nov 2010 |
|
WO |
|
2011/136175 |
|
Nov 2011 |
|
WO |
|
Other References
Key to Steel, 10th Edition 1974, Verlag Stahlschlussel , West
Germany. cited by examiner.
|
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Clark & Brody
Claims
The invention claimed is:
1. A stainless steel for oil wells comprising, by mass %, C: not
more than 0.05%, Si: not more than 1.0%, Mn: 0.01 to 1.0%, P: not
more than 0.05%, S: less than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%,
Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0%, Al: 0.001 to
0.1%, O: not more than 0.05%, and N: not more than 0.05%, the
balance being Fe and impurities, and satisfying Formulas (1) and
(2): Cr+4Ni+3Mo+2Cu.gtoreq.44 (1) Cr+3Ni+4Mo+2Cu/3.ltoreq.46 (2)
where each symbol of element in Formulas (1) and (2) is substituted
by a content, in mass %, of a corresponding element.
2. The stainless steel for oil wells according to claim 1, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
V: not more than 0.3%, Ti: not more than 0.3%, Nb: not more than
0.3%, and Zr: not more than 0.3%.
3. The stainless steel for oil wells according to claim 1, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
W: not more than 1.0%, and rare earth metal (REM): not more than
0.3%.
4. The stainless steel for oil wells according to claim 2, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
W: not more than 1.0%, and rare earth metal (REM): not more than
0.3%.
5. The stainless steel for oil wells according to claim 1, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
Ca: not more than 0.01%, and B: not more than 0.01%.
6. The stainless steel for oil wells according to claim 2, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
Ca: not more than 0.01%, and B: not more than 0.01%.
7. The stainless steel for oil wells according to claim 3, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
Ca: not more than 0.01%, and B: not more than 0.01%.
8. The stainless steel for oil wells according to claim 4, wherein
the stainless steel for oil wells contains, in place of some of Fe,
one or more kinds of elements selected from the group consisting of
Ca: not more than 0.01%, and B: not more than 0.01%.
9. The stainless steel for oil wells according to claim 1, wherein
a metal micro-structure of the stainless steel for oil wells
contains, by volume ratio, not less than 10% and less than 60% of
ferrite phase, not more than 10% of retained austenite phase, and
not less than 40% of martensite phase.
10. The stainless steel for oil wells according to any one of
claims 1 to 8, wherein the stainless steel for oil wells has a
yield strength of not less than 862 MPa.
11. The stainless steel for oil wells according to claim 9, wherein
the stainless steel for oil wells has a yield strength of not less
than 862 MPa.
12. An oil well pipe manufactured from the stainless steel for oil
wells according to claim 1.
13. An oil well pipe manufactured from the stainless steel for oil
wells according to claim 9.
14. An oil well pipe manufactured from the stainless steel for oil
wells according to claim 10.
15. An oil well pipe manufactured from the stainless steel for oil
wells according to claim 11.
Description
TECHNICAL FIELD
The present invention relates to a stainless steel for oil wells
and a stainless steel pipe for oil wells, and more particularly to
a stainless steel for oil wells and a stainless steel pipe for oil
wells, which are used in a high-temperature oil well environment
and gas well environment (hereinafter, referred to as a
high-temperature environment).
BACKGROUND ART
In the present description, an oil well and a gas well are
collectively referred to simply as "an oil well". Accordingly, "a
stainless steel for oil wells" as used herein includes a stainless
steel for oil wells and a stainless steel for gas well. Also "a
stainless steel pipe for oil wells" includes a stainless steel pipe
for oil wells and a stainless steel pipe for gas well.
As used herein, the term "a high temperature" means, unless
otherwise stated, a temperature not less than 150.degree. C. Also
as used herein, the symbol "%" relating to a chemical element
means, unless otherwise stated, "mass %".
A conventional oil well environment contains carbon dioxide gas
(CO.sub.2) and/or chlorine ion (Cl.sup.-). For that reason, in a
conventional oil well environment, a martensitic stainless steel
containing 13% of Cr (hereafter, referred to as a "13% Cr steel")
is commonly used. The 13% Cr steel is excellent in carbonic-acid
gas corrosion resistance.
Recently, the development of deep oil wells has advanced. A deep
oil well has a high-temperature environment. Such high-temperature
environment includes carbon dioxide gas or carbon dioxide gas and
hydrogen sulfide gas. These gases are corrosive gases. Therefore,
steel for oil wells to be used in deep oil wells is required to
have a higher strength and a higher corrosion resistance than those
of the 13% Cr steel.
The Cr content of a two-phase stainless steel is greater than that
of the 13% Cr steel. Therefore, a two-phase stainless steel has a
higher strength and a higher corrosion resistance than those of the
13% Cr steel. The two-phase stainless steel is, for example, a 22%
Cr steel containing 22% of Cr, and a 25% Cr steel containing 25% of
Cr. Although the two-phase stainless steel has a high strength and
a high corrosion resistance, it includes many alloy elements, and
therefore is expensive.
JP2002-4009A, JP2005-336595A, JP2006-16637A, JP2007-332442A,
WO2010/050519, and WO2010/134498 propose stainless steels other
than the above described two-phase stainless steel. The stainless
steels disclosed in these literatures contain at the maximum 17 to
18.5% of Cr.
JP2002-4009A proposes a martensitic stainless steel for oil wells,
which has a yield strength of not less than 860 MPa and a
carbonic-acid gas corrosion resistance in a high-temperature
environment. The chemical composition of the stainless steel
disclosed in this literature contains 11.0 to 17.0% of Cr and 2.0
to 7.0% of Ni, and further satisfies: Cr+Mo+0.3
Si-40C-10N-Ni-0.3Mn.ltoreq.10. The metal micro-structure of this
stainless steel is predominantly made up of martensite, and
contains not more than 10% of a retained austenite.
JP2005-336595A proposes a stainless steel pipe which has a high
strength and carbonic-acid gas corrosion resistance in a
high-temperature environment of 230.degree. C. The chemical
composition of the stainless steel pipe disclosed in this
literature contains 15.5 to 18% of Cr, 1.5 to 5% of Ni, and 1 to
3.5% of Mo, satisfies Cr+0.65Ni+0.6Mo+0.55Cu-20C.gtoreq.19.5 and
also satisfies Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N.gtoreq.11.5.
The metal micro-structure of this stainless steel pipe contains 10
to 60% of a ferrite phase, and not more than 30% of an austenite
phase, the balance being a martensite phase.
JP 2006-16637A proposes a stainless steel pipe which has a high
strength and carbonic-acid gas corrosion resistance in a
high-temperature environment of more than 170.degree. C. The
chemical composition of the stainless steel pipe disclosed in this
literature contains 15.5 to 18.5% of Cr, and 1.5 to 5% of Ni,
satisfies Cr+0.65 Ni+0.6 Mo+0.55Cu-20C.gtoreq.18.0 and also
satisfies Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N.gtoreq.11.5. The
metal micro-structure of this stainless steel pipe may or may not
include an austenite phase.
JP 2007-332442A proposes a stainless steel pipe which has a high
strength of not less than 965 MPa, and a carbonic-acid gas
corrosion resistance in a high-temperature environment exceeding
170.degree. C. The chemical composition of the stainless steel pipe
disclosed in this literature contains, by mass %, 14.0 to 18.0% of
Cr, 5.0 to 8.0% of Ni, 1.5 to 3.5% of Mo, and 0.5 to 3.5% Cu, and
satisfies Cr+2 Ni+1.1 Mo+0.7 Cu.ltoreq.32.5. The metal
micro-structure of this stainless steel pipe contains 3 to 15% of
an austenite phase, the balance being a martensite phase.
WO2010/050519 proposes a stainless pipe which has a sufficient
corrosion resistance even in a high-temperature carbon dioxide
environment of 200.degree. C., and further has a sufficient sulfide
stress-corrosion cracking resistance even when the environment
temperature of an oil well or gas well declines due to a temporary
suspension of the collection of crude oil or gas. The chemical
composition of the stainless steel pipe disclosed in this
literature contains more than 16% and not more than 18% of Cr, more
than 2% and not more than 3% of Mo, not less than 1% and not more
than 3.5% of Cu, and not less than 3% and less than 5% of Ni, while
Mn and N satisfy [Mn].times.([N]-0.0045).ltoreq.0.001. The metal
micro-structure of this stainless steel pipe contains 10 to 40% by
volume fraction of a ferrite phase, and not more than 10% by volume
fraction of a retained .gamma. phase with a martensite phase being
as the dominant phase.
WO2010/134498 proposes a high-strength stainless steel which has an
excellent corrosion resistance in a high-temperature environment,
and has an SSC resistance (sulfide stress-corrosion cracking
resistance) at normal temperature. The chemical composition of the
stainless steel disclosed in this literature contains more than 16%
and not more than 18% of Cr, not less than 1.6% and not more than
4.0% of Mo, not less than 1.5% and not more than 3.0% of Cu, and
more than 4.0% and not more than 5.6% of Ni, and satisfies
Cr+Cu+Ni+Mo.gtoreq.25.5 and
-8.ltoreq.30(C+N)+0.5Mn+Ni+Cu/2+8.2-1.1(Cr+Mo).ltoreq.-4. The metal
micro-structure of this stainless steel contains a martensite
phase, 10 to 40% of a ferrite phase, and a retained austenite
phase, with a ferrite phase distribution rate being higher than
85%.
DISCLOSURE OF THE INVENTION
However, in the stainless steels disclosed in the above described
patent literatures, it is not necessarily easy to stably obtain a
desired metal micro-structure, and there may be a case where a
desired yield strength is not stably obtained. In the industrial
production of stainless steel, time spent for a heat treatment
process and a cooling process will be limited in order to improve
productivity. Therefore, there may be a case where a high strength
not less than 758 MPa is not stably obtained.
It is an object of the present invention to provide a stainless
steel for oil wells, which has an excellent high-temperature
corrosion resistance and can stably obtain a strength of not less
than 758 MPa.
A stainless steel for oil wells of the present invention contains,
by mass %, C: not more than 0.05%, Si: not more than 1.0%, Mn: 0.01
to 1.0%, P: not more than 0.05%, S: less than 0.002%, Cr: 16 to
18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to
1.0%, Al: 0.001 to 0.1%, O: not more than 0.05%, and N: not more
than 0.05%, the balance being Fe and impurities, and satisfies
Formulas (1) and (2): Cr+4Ni+3Mo+2Cu.gtoreq.44 (1)
Cr+3Ni+4Mo+2Cu/3.ltoreq.46 (2)
where each symbol of element in Formulas (1) and (2) is substituted
by the content (mass %) of a corresponding element.
The above described stainless steel for oil wells may contain, in
place of some of Fe, one or more kinds of elements selected from
the group consisting of V: not more than 0.3%, Ti: not more than
0.3%, Nb: not more than 0.3%, and Zr: not more than 0.3%. The above
described stainless steel for oil wells may contain, in place of
some of Fe, one or more kinds of elements selected from the group
consisting of W: not more than 1.0%, and rare earth metal (REM):
not more than 0.3%. The above described stainless steel for oil
wells may contain, in place of some of Fe, one or more kinds of
elements selected from the group consisting of Ca: not more than
0.01%, and B: not more than 0.01%.
The metal micro-structure of the above described stainless steel
preferably contains, by volume ratio, not less than 10% and less
than 60% of a ferrite phase, not more than 10% of a retained
austenite phase, and not less than 40% of a martensite phase.
The stainless steel pipe for oil wells according to the present
invention is manufactured from the above described stainless steel
for oil wells.
The stainless steel pipe for oil wells according to the present
invention has a high strength and an excellent high-temperature
corrosion resistance and can stably obtain high strength.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereafter, embodiments of the present invention will be described
in detail. The present inventors have conducted a survey and
analysis and consequently obtained the following findings.
(A) To obtain a stress corrosion cracking resistance (SCC
resistance) in a high-temperature environment, it is preferable
that Ni, Mo, and Cu besides Cr are contained. To be more specific,
an excellent SCC resistance will be obtained in a high-temperature
environment when the following Formula (1) is satisfied:
Cr+4Ni+3Mo+2Cu.gtoreq.44 (1)
where each symbol of element in Formula (1) is substituted by the
content (mass %) of a corresponding element.
(B) When the contents of alloy elements such as Cr, Ni, Mo, and Cu
increase, it is not likely that a high strength is stably obtained.
The variation of strength will be suppressed, and yield strength of
not less than 758 MPa will be stably obtained when the following
Formula (2) is satisfied: Cr+3Ni+4Mo+2Cu/3.ltoreq.46 (2)
where each symbol of element in Formula (2) is substituted by the
content (mass %) of a corresponding element.
(C) Co stabilizes the strength and corrosion resistance. When
Formulas (1) and (2) are satisfied, and 0.01 to 1.0% of Co is
contained, a stable metal micro-structure will be obtained, and a
stable and high strength and an excellent corrosion resistance in a
high-temperature environment will be obtained.
The present invention has been completed based on the above
described findings. Hereafter, details of the stainless steel for
oil wells of the present invention will be described.
[Chemical Composition]
The stainless steel for oil wells according to the present
invention has the following chemical composition.
C: not more than 0.05%
Although carbon (C) contributes to increase strength, it produces
carbide at the time of tempering. Cr carbide deteriorates the
corrosion resistance to high-temperature carbon dioxide gas.
Therefore, the C content is preferably smaller. The C content is
not more than 0.05%. Preferably the C content is less than 0.05%,
more preferably not more than 0.03%, and even more preferably not
more than 0.01%.
Si: not more than 1.0%
Silicon (Si) deoxidizes steel. However, an excessive Si content
will deteriorate hot workability. Moreover, it increases the amount
of ferrite to be produced, thereby reducing yield strength (yield
stress). Therefore, the Si content is not more than 1.0%.
Preferably the Si content is not more than 0.8%, more preferably
not more than 0.5%, and even more preferably not more than 0.4%.
When the Si content is not less than 0.05%, Si acts in a
particularly effective manner as a deoxidizer. However, even when
the Si content is less than 0.05%, Si deoxidizes steel to some
extent.
Mn: 0.01 to 1.0%
Manganese (Mn) deoxidizes and desulfurizes steel, thereby improving
hot workability. However, an excessive Mn content is likely to
cause segregations in steel, thereby deteriorating the toughness
and the SCC resistance in a high-temperature chloride aqueous
solution. Moreover, Mn is an austenite forming element. Therefore,
when steel contains Ni and Cu which are austenite forming elements,
an excessive Mn content will lead to an increase of retained
austenite, thereby reducing the yield strength (yield stress).
Therefore, the Mn content is 0.01 to 1.0%. The lower limit of Mn
content is preferably 0.03%, more preferably 0.05%, and even more
preferably 0.07%. The upper limit of Mn content is preferably 0.5%,
more preferably less than 0.2%, and even more preferably 0.14%.
P: not more than 0.05%
Phosphor (P) is an impurity. P deteriorates the sulfide
stress-corrosion cracking resistance (SSC resistance) and the SCC
resistance in a high-temperature chloride aqueous solution
environment of steel. Therefore, the P content is preferably as low
as possible. The P content is not more than 0.05%. Preferably the P
content is less than 0.05%, more preferably not more than 0.025%,
and even more preferably not more than 0.015%.
S: less than 0.002%
Sulfur (S) is an impurity. S deteriorates hot workability of steel.
The metal micro-structure of a stainless steel of the present
invention becomes a two-phase micro-structure including a ferrite
phase and an austenite phase during hot working. S deteriorates hot
workability of such two-phase micro-structure. Further, S combines
with Mn etc. to form inclusions. The inclusions formed act as a
starting point of pitting and SCC, thereby deteriorating the
corrosion resistance of steel. Therefore, the S content is
preferably as low as possible. The S content is less than 0.002%.
Preferably the S content is not more than 0.0015%, and more
preferably not more than 0.001%.
Cr: 16 to 18%
Chromium (Cr) improves the SCC resistance in a high-temperature
chloride aqueous solution environment. However, since Cr is a
ferrite forming element, an excessive Cr content will lead to an
excessive increase in the amount of ferrite in steel, thereby
deteriorating the yield strength of steel. Therefore, Cr content is
16 to 18%. The lower limit of Cr content is preferably more than
16%, more preferably 16.3%, and even more preferably 16.5%. The
upper limit of Cr content is preferably less than 18%, more
preferably 17.8%, and even more preferably 17.5%.
Mo: 1.8 to 3%
When the production of fluid is temporarily stopped in an oil well,
the temperature of the fluid in an oil well pipe will decline. At
this moment, the sulfide stress-corrosion cracking susceptibility
of a high-strength material generally increases. Molybdenum (Mo)
improves the sulfide stress-corrosion cracking susceptibility.
Further, Mo improves the SCC resistance of steel under coexistence
with Cr. However, since Mo is a ferrite forming element, an
excessive Mo content will lead to an increase of the amount of
ferrite in steel, thereby reducing the strength of steel.
Therefore, the Mo content is 1.8 to 3%. The lower limit of Mo
content is preferably more than 1.8%, more preferably 2.0%, and
even more preferably 2.1%. The upper limit of Mo content is
preferably less than 3%, more preferably 2.7%, and even more
preferably 2.6%.
Cu: 1.0 to 3.5%
Cupper (Cu) strengthens a ferrite phase by age precipitation,
thereby increasing the strength of steel. Further, Cu reduces the
dissolution rate of steel in a high-temperature chloride aqueous
solution environment, thereby improving the corrosion resistance of
steel. However, an excessive Cu content will lead to a
deterioration of the hot workability of steel, thereby
deteriorating the toughness of steel. Therefore, the Cu content is
1.0 to 3.5%. The lower limit of Cu content is preferably more than
1.0%, more preferably 1.5%, and even more preferably 2.2%. The
upper limit of Cu content is less than 3.5%, more preferably 3.2%,
and even more preferably 3.0%.
Ni: 3.0 to 5.5%
Since nickel (Ni) is an austenite forming element, it stabilizes
austenite at high temperature and increases the amount of
martensite at normal temperature. Therefore, Ni increases the
strength of steel. Further, Ni improves the corrosion resistance in
a high-temperature chloride aqueous solution environment. However,
an excessive Ni content tends to lead to an increase of retained
.gamma. phase, and it becomes difficult to stably obtain a high
strength especially at the time of industrial production.
Therefore, the Ni content is 3.0 to 5.5%. The lower limit of Ni
content is preferably more than 3.0%, more preferably 3.5%, even
more preferably 4.0%, and even more preferably 4.2%. The upper
limit of Ni content is preferably less than 5.5%, more preferably
5.2%, and even more preferably 4.9%.
Co: 0.01 to 1.0%
Cobalt (Co) improves the hardenability of steel, and ensures a
stable and high strength especially at the time of industrial
production. To be more specific, Co suppresses retained austenite,
thereby suppressing the variation of strength. However, an
excessive Co content will lead to a deterioration of the toughness
of steel. Therefore, the Co content is 0.01 to 1.0%. The lower
limit of Co content is preferably more than 0.01%, more preferably
0.02%, even more preferably 0.1%, and even more preferably 0.25%.
The upper limit of Co content is preferably less than 1.0%, more
preferably 0.95%, and even more preferably 0.75%.
Al: 0.001 to 0.1%
Aluminum (Al) deoxidizes steel. However, an excessive Al content
will lead to an increase of the amount of ferrite in steel, thereby
deteriorating the strength of steel. Further, a large amount of
alumina-based inclusions are produced in steel, thereby
deteriorating the toughness of steel. Therefore, the Al content is
0.001 to 0.1%. The lower limit of Al content is preferably more
than 0.001%, and more preferably 0.01%. The upper limit of Al
content is preferably less than 0.1%, and more preferably
0.06%.
As used herein, the term "Al content" means the content of
acid-soluble Al (sol. Al).
O (Oxygen): not more than 0.05%
Oxygen (O) deteriorates the toughness and corrosion resistance of
steel. Therefore, the O content is preferably lower. The O content
is not more than 0.05%. Preferably the O content is less than
0.05%, more preferably not more than 0.01%, and even more
preferably not more than 0.005%.
N: not more than 0.05%
Nitrogen (N) increases the strength of steel. Further, N stabilizes
austenite, thereby improving pitting resistance. When even a small
amount of N is contained, the above described effects can be
obtained to some extent. On the other hand, an excessive N content
will lead to a production of a large amount of nitrides in steel,
thereby deteriorating the toughness of steel. Further, austenite
becomes more likely to be retained, thereby reducing the strength
of steel. Therefore, the N content is not more than 0.05%. The
lower limit of N content is preferably 0.002%, and more preferably
0.005%. The upper limit of N content is preferably 0.03%, more
preferably 0.02%, even more preferably 0.015%, and even more
preferably 0.010%.
The balance of the chemical composition of a stainless steel for
oil wells is made up of impurities. The term "an impurity" as used
herein refers to an element which is mixed from ores and scraps
which are used as the starting material of steel, or the
environments in the manufacturing process, etc.
[Regarding Selective Elements]
An stainless steel for oil wells may further contain, in place of
some of Fe, one or more kinds of elements selected from the group
consisting of V: not more than 0.3%, Ti: not more than 0.3%, Nb:
not more than 0.3%, and Zr: not more than 0.3%.
V: not more than 0.3%,
Nb: not more than 0.3%,
Ti: not more than 0.3%, and
Zr: not more than 0.3%.
Vanadium (V), niobium (Nb), titanium (Ti), and zirconium (Zr) are
all selective elements. Any of these elements forms carbide and
increases the strength and toughness of steel. Further, these
elements immobilize C and thereby suppress Cr carbide from being
produced. For that reason, the pitting resistance of steel is
improved, and the SCC susceptibility is reduced. When these
elements are contained even in a small amount, the above described
effects are obtained to some extent. On the other hand, when the
contents of these elements are excessively large, carbides are
coarsened and thereby the toughness and the corrosion resistance of
steel deteriorate. Therefore, the V content, Nb content, Ti
content, and Zr content are not more than 0.3%, respectively. The
lower limits of V, Nb, Ti, and Zr are preferably 0.005%,
respectively. The upper limits of V, Nb, Ti, and Zr are preferably
less than 0.3%, respectively.
A stainless steel for oil wells may contain, in place of some of
Fe, one or more kinds of elements selected from the group
consisting of W: not more than 1.0% and rare earth metal (REM): not
more than 0.3%.
W: not more than 1.0%
REM: not more than 0.3%
Tungsten (W) and rare earth metal (REM) are both selective
elements. Herein, the term "REM" refers to one or more kinds of
elements selected from the group consisting of yttrium (Y) of
atomic number 39, lanthanum (La) of atomic number 57 to lutetium
(Lu) of atomic number 71 which are lanthanoid elements, and
actinium (Ac) of atomic number 89 to lawrencium (Lr) of atomic
number 103, which are actinoid elements.
W and REM both improve the SCC resistance in a high-temperature
environment. When these elements are contained even in a small
amount, the above described effect will be achieved to some extent.
On the other hand, when the contents of these elements are
excessively large, the effects thereof will be saturated.
Therefore, the W content is not more than 1.0% and the REM content
is not more than 0.3%. When REM includes a plurality of elements
selected from the above described group, the REM content means a
total content of those elements. The lower limit of W content is
preferably 0.01%. The lower limit of REM content is preferably
0.001%.
A stainless steel for oil wells may contain, in place of some of
Fe, one or more kinds of elements selected from the group
consisting of Ca: not more than 0.01% and B: not more than
0.01%.
Ca: not more than 0.01%
B: not more than 0.01%
Calcium (Ca) and boron (B) are both selective elements. A stainless
steel for oil wells during hot working has a two-phase
micro-structure of ferrite and austenite. For that reason, flaws
and defects may be produced in the stainless steel due to hot
working. Ca and B suppress flaws and defects from been produced
during hot working. When these elements are contained even in a
small amount, the above described effect will be obtained to some
extent.
On the other hand, an excessive Ca content will lead to an increase
of inclusions in steel, thereby deteriorating the toughness and
corrosion resistance of steel. Further, an excessive B content will
lead to a precipitation of carbo-boride at grain boundaries,
thereby deteriorating the toughness of steel. Therefore, the Ca
content and B content are both not more than 0.01%.
The lower limits of Ca content and B content are both preferably
0.0002%. In this case, the above described effect will be
remarkably obtained. The upper limits of Ca content and B content
are both preferably less than 0.01%, and are both more preferably
0.005%.
[Regarding Formulas (1) and (2)]
The chemical composition of the stainless steel for oil wells
further satisfies Formulas (1) and (2): Cr+4Ni+3Mo+2Cu.gtoreq.44
(1) Cr+3Ni+4Mo+2Cu/3.ltoreq.46 (2)
where each symbol of element in Formulas (1) and (2) is substituted
by the content (%) of a corresponding element.
[Regarding Formula (1)]
Definition is made as F1=Cr+4Ni+3Mo+2Cu. As F1 increases, the SCC
resistance in a high-temperature oil well environment will be
improved. When the value of F1 is not less than 44, an excellent
SCC resistance will be obtained in a high-temperature oil well
environment of 150.degree. C. to 200.degree. C. The value of F1 is
preferably not less than 45, and more preferably not less than 48.
A sufficient SCC resistance at room temperature is also ensured if
the value of F1 is not less than 44.
The upper limit of the value of F1 will not be particularly
limited. However, when the value of F1 exceeds 52, it becomes
difficult to satisfy Formula (2), and thereby the stability of
yield strength deteriorates.
[Regarding Formula (2)]
A definition is made as F2=Cr+3Ni+4Mo+2Cu/3. In the stainless steel
pipe for oil wells of the present invention, the above described Co
is contained and the value of F2 is made not more than 46 to stably
secure the strength. When the value of F2 exceeds 46, a retained
austenite is excessively formed, and it becomes difficult to stably
secure the yield strength.
The value of F2 is preferably not more than 44, more preferably not
more than 43, and even more preferably not more than 42. The lower
limit of the value of F2 is not particularly limited. However, when
the value of F2 is not more than 36, there will be a case where the
value of F1 is not likely to become not less than 44.
[Relation Between C and N]
The chemical composition of a stainless steel for oil wells
preferably satisfies Formula (3): 2.7C+N.ltoreq.0.060 (3)
where C and N in Formula (3) are substituted by the C content (%)
and N content (%), respectively.
A definition is made as F3=2.7C+N. When the value of F3 is not more
than 0.060, a retained austenite is further suppressed from being
produced. Therefore, combined with the effect of Formula (2), it is
possible to secure the strength more stably. The value of F3 is
preferably not more than 0.050, and more preferably not more than
0.045.
[Metal Micro-Structure]
The metal micro-structure of a stainless steel for oil wells
preferably contains, by volume ratio, less than 10 to 60% of a
ferrite phase, not more than 10% of a retained austenite phase, and
a martensite phase.
Ferrite phase: not less than 10% and less than 60% by volume
ratio
The stainless steel for oil wells of the present invention has
large contents of Cr and Mo which are ferrite forming elements. On
the other hand, although Ni is contained in the view point of
stabilizing austenite at high temperature and securing martensite
at normal temperature, the content of Ni which is an austenite
forming element, is suppressed to a level at which the amount of
retained austenite is not excessive. Therefore, the stainless steel
of the present invention will not be a martensite single-phase
micro-structure at normal temperature, and will be a mixed
micro-structure including at least a martensite phase and a ferrite
phase at normal temperature. While a martensite phase in the metal
micro-structure contributes to an increase in strength, an
excessive volume ratio of ferrite phase will deteriorate the
strength of steel. Therefore, the volume ratio of ferrite phase is
preferably not less than 10% and less than 60%. The lower limit of
the volume ratio of ferrite phase is preferably more than 10%, more
preferably 12%, and even more preferably 14%. The upper limit of
the volume ratio of ferrite phase is preferably 48%, more
preferably 45%, and even more preferably 40%.
The volume ratio of ferrite phase is determined by the following
method. A sample is taken from an arbitrary location of a stainless
steel. In the sample taken, a sample surface which corresponds to a
cross section of the stainless steel is ground. After grinding, the
ground sample surface is etched by using a mixed solution of aqua
regia and glycerin. The area fraction of ferrite phase on the
etched surface is measured by a point counting method conforming to
JIS G0555 by using an optical microscope (observation
magnifications of 100). The measured area fraction is defined as a
volume ratio of ferrite phase.
Retained austenite phase: not more than 10% by volume ratio
A small amount of retained austenite will not cause a remarkable
decline of strength, and will remarkably improve the toughness of
steel. However, an excessive volume ratio of retained austenite
will lead to a remarkable decline of the strength of steel.
Therefore, the volume ratio of retained austenite phase is not more
than 10%. From view point of securing strength, a more preferable
volume ratio of retained austenite phase is not more than 8%.
When the volume ratio of retained austenite phase is not less than
0.5%, the above described effect of improving toughness will be
obtained effectively. However, even if the volume ratio of retained
austenite phase is less than 0.5%, the above described effect will
be obtained to some extent.
The volume ratio of retained austenite phase is determined by an
X-ray diffraction method. To be specific, a sample is taken from an
arbitrary location of a stainless steel. The size of the sample is
15 mm.times.15 mm.times.2 mm. Respective X ray intensities of the
(200) and (211) planes of ferrite phase (.alpha. phase), and (200),
(220), and (311) planes of retained austenite phase (.gamma. phase)
are measured by using a sample. Then, the integrated intensity of
each plane is calculated. After the calculation, a volume ratio of
retained austenite phase V.gamma.(%) is calculated for each of
combinations (a total of 6 combinations) of each plane of the
.alpha. phase and each plane of the .gamma. phase by using Formula
(1). Then, an average value of volume ratios V.gamma. of 6
combinations is defined as the volume ratio (%) of retained
austenite.
V.gamma.=100/(1+(I.alpha..times.R.gamma.)/(I.gamma..times.R.alpha.))
(1)
Where "I.alpha." is the integrated intensity of .alpha. phase.
"R.alpha." is a crystallographic theoretical calculation value of
.alpha. phase. "I.gamma." is the integrated intensity of .gamma.
phase. "R.gamma." is a crystallographic theoretical calculation
value of .gamma. phase.
Martensite phase: Balance
In the metal micro-structure of a stainless steel of the present
invention, the portions other than the above described ferrite
phase and the retained austenite phase are predominantly a tempered
martensite phase. To be more specific, the metal micro-structure of
the stainless steel of the present invention preferably contains
not less than 40% by volume ratio of a martensite phase. The lower
limit of the volume ratio of martensite is more preferably 48%, and
even more preferably 52%. The volume ratio of martensite phase is
determined by subtracting the volume ratios of ferrite phase and
retained austenite phase, which are determined by the above
described method, from 100%.
The metal micro-structure of a stainless steel for oil wells may
contain precipitates and/or inclusions such as carbides, nitrides,
borides, and a Cu phase besides a ferrite phase, a retained
austenite phase, and a martensite phase.
[Manufacturing Method]
A method for manufacturing a seamless steel pipe will be described
as one example of a method for manufacturing a stainless steel for
oil wells.
A starting material having the above described chemical composition
is prepared. The starting material may be a cast piece manufactured
by a continuous casting method (including a round CC). Moreover, it
may be a billet manufactured by hot working an ingot manufactured
by an ingot-making process. It may also be a billet manufactured
from the cast piece.
The prepared starting material is charged into a reheating furnace
or a soaking pit to be heated. Next, the heated starting material
is subjected to hot working to manufacture a hollow shell. For
example, a Mannesmann process is performed as hot working. To be
specific, the starting material is piercing-rolled by a piercing
machine to be formed into a hollow shell. Next, the hollow shell is
further rolled, for example, by a mandrel mill and a sizing mill.
As hot working, hot extrusion may be performed, or hot forging may
be performed.
It is preferable that the reduction of area of a starting material
while the temperature of the starting material is 850 to
1250.degree. C. is not less than 50% during hot working. In the
range of the chemical composition of the steel of the present
invention, performing hot working such that the reduction of area
of the starting material while the temperature of the starting
material is 850 to 1250.degree. C. is not less than 50% will result
in that a micro-structure including a martensite phase and a
ferrite phase which is long-stretched (for example, about 50 to 200
.mu.m) in the rolling direction is formed in the near-surface
portion of steel. Since a ferrite phase is more likely to contain
Cr etc. than a martensite, it effectively contributes to the
prevention of the propagation of SCC at high temperature. As so far
described, when the ferrite phase is long-stretched in the rolling
direction, even if SCC occurs on the surface at high temperature,
it becomes more likely to reach the ferrite phase during the course
of the propagation of crack. For this reason, SCC resistance at
high temperature improves.
The hollow shell after hot working is cooled to normal temperature.
The cooling method may be either air cooling or water cooling.
Since in a stainless steel of the present invention, martensite
transformation will occur when it is cooled to or lower than a Ms
point even by air cooling, it is possible to obtain a mixed
micro-structure including martensite and ferrite. However, when
attempting to stably secure a high strength of not less than 758
MPa, particularly a high strength of not less than 862 MPa, it is
preferable that the hot rolled hollow shell is air cooled,
thereafter reheated to not lower than an A.sub.c3 transformation
point, and is quenched by performing water cooling such as a
dipping method and a spray method.
Although decreasing the value of F2 or increasing the Co content
may make it possible to obtain a high strength even by air cooling,
there may be a lack of stability in strength. To stably obtain a
high strength, the steel is cooled by water cooling till the
surface temperature of the hollow shell becomes not more than
60.degree. C. That is, the hollow shell after hot working is
preferably water cooled and a water-cooling stopping temperature is
made not more than 60.degree. C. The water-cooling stopping
temperature is more preferably not more than 45.degree. C., and
even more preferably not more than 30.degree. C.
The quenched hollow shell is tempered at not more than an A.sub.c1
point so that the yield strength is adjusted to be not less than
758 MPa. When the tempering temperature exceeds the A.sub.c1 point,
the volume ratio of retained austenite sharply increases, and the
strength deteriorates.
The high-strength stainless steel for oil wells manufactured by the
above described processes has a yield stress of not less than 758
MPa, and has an excellent corrosion resistance even in a
high-temperature oil well environment of 200.degree. C. owing to
the effects of Cr, Mo, Ni, and Cu contained therein.
EXAMPLES
Steels of marks 1 to 28 having chemical compositions shown in Table
1 were melted, and cast pieces were manufactured by a continuous
casting.
TABLE-US-00001 TABLE 1 Chemical Composition (in mass %, the balance
being Fe and impurities) Mark C Si Mn P S Cr Mo Cu Ni Co Al O N 1
0.008 0.20 0.11 0.012 0.0012 16.28 2.82 3.44 4.85 0.230 0.040
0.0017 0.- 0122 2 0.013 0.26 0.20 0.010 0.0012 17.41 2.34 1.32 5.44
0.202 0.045 0.0020 0.- 0151 3 0.008 0.45 0.09 0.015 0.0008 17.02
1.98 3.38 3.65 0.628 0.033 0.0016 0.- 0100 4 0.011 0.32 0.08 0.023
0.0010 17.66 2.43 2.67 5.17 0.073 0.038 0.0021 0.- 0099 5 0.017
0.23 0.07 0.019 0.0012 16.50 2.53 3.35 4.38 0.156 0.039 0.0017 0.-
0211 6 0.032 0.42 0.29 0.017 0.0014 16.75 2.12 1.90 4.50 0.185
0.051 0.0016 0.- 0102 7 0.016 0.40 0.14 0.018 0.0012 17.50 2.69
2.60 4.99 0.050 0.032 0.0011 0.- 0084 8 0.031 0.47 0.10 0.012
0.0014 17.32 2.61 3.12 4.48 0.072 0.027 0.0014 0.- 0090 9 0.019
0.30 0.07 0.015 0.0012 16.20 2.81 3.22 4.30 0.150 0.037 0.0016 0.-
0084 10 0.018 0.33 0.32 0.017 0.0013 17.10 2.40 3.30 4.50 0.130
0.033 0.0015 0.- 0124 11 0.012 0.37 0.09 0.012 0.0014 16.08 2.04
2.72 4.24 0.110 0.028 0.0018 0.- 0077 12 0.025 0.27 0.08 0.011
0.0010 16.57 2.07 2.94 4.38 0.340 0.038 0.0015 0.- 0090 13 0.022
0.25 0.09 0.018 0.0013 16.26 2.37 2.71 4.70 0.520 0.026 0.0010 0.-
0079 14 0.016 0.39 0.07 0.010 0.0018 17.75 2.02 1.88 5.01 0.240
0.027 0.0019 0.- 0085 15 0.017 0.27 0.08 0.014 0.0009 17.17 2.06
2.39 4.12 0.014 0.029 0.0015 0.- 0084 16 0.007 0.34 0.15 0.019
0.0015 16.52 2.34 2.26 4.50 0.031 0.032 0.0020 0.- 0080 17 0.026
0.35 0.12 0.014 0.0012 17.22 2.43 1.91 5.30 0.189 0.038 0.0016 0.-
0072 18 0.018 0.27 0.06 0.012 0.0017 17.50 2.15 3.32 4.25 0.152
0.044 0.0018 0.- 0095 19 0.010 0.32 0.09 0.014 0.0016 16.82 2.61
2.89 4.20 0.173 0.043 0.0012 0.- 0073 20 0.014 0.34 0.09 0.016
0.0013 16.57 2.19 2.41 4.92 0.058 0.026 0.0010 0.- 0090 21 0.022
0.30 0.55 0.011 0.0011 17.30 2.90 1.71 4.75 --* 0.039 0.0020 0.00-
77 22 0.025 0.25 0.35 0.015 0.0017 17.50 2.76 2.55 4.98 0.005*
0.025 0.0023 0- .0093 23 0.013 0.26 0.15 0.014 0.0015 16.41 2.52
1.79 4.47 1.211* 0.025 0.0019 0- .0083 24 0.023 0.32 0.48 0.013
0.0015 16.20 2.20 2.85 3.30 0.097 0.020 0.0011 0.- 0113 25 0.017
0.28 0.19 0.017 0.0016 17.60 2.80 3.40 5.30 0.050 0.036 0.0016 0.-
0091 26 0.015 0.33 0.11 0.012 0.0013 17.52 2.92 3.41 5.42 0.032
0.035 0.0017 0.- 0078 27 0.012 0.27 0.15 0.013 0.0013 17.62 2.77
3.45 5.39 0.017 0.041 0.0025 0.- 0063 28 0.025 0.47 0.58 0.012
0.0014 17.55 2.88 3.38 5.10 0.250 0.027 0.0014 0.- 0090 Chemical
Composition (in mass %, the balance being Fe and impurities) Mark V
Ti Nb Zr W REM Ca B F1 F2 F3 1 -- -- -- -- -- -- -- -- 51.0 44.4
0.033 2 -- -- -- -- -- -- -- -- 48.8 44.0 0.051 3 -- -- -- -- -- --
-- -- 44.3 38.1 0.033 4 -- -- -- -- -- -- -- -- 51.0 44.7 0.039 5
-- -- -- -- -- -- -- -- 48.3 42.0 0.067 6 -- -- -- -- -- -- -- --
44.9 40.0 0.097 7 -- -- -- -- -- -- -- -- 50.7 45.0 0.053 8 -- --
-- -- -- -- -- -- 49.3 43.3 0.093 9 0.28 -- -- -- -- -- -- -- 48.3
42.5 0.059 10 0.26 -- -- -- -- -- -- 48.9 42.4 0.061 11 0.17 --
0.24 -- -- -- -- -- 44.6 38.8 0.039 12 -- -- -- 0.25 0.47 -- -- --
46.2 39.9 0.077 13 -- -- -- -- -- -- -- -- 47.6 41.7 0.067 14 --
0.18 0.15 -- -- La: 0.23 -- -- 47.6 42.1 0.051 15 -- -- -- -- -- --
0.008 -- 44.6 39.4 0.055 16 -- -- -- -- -- -- 0.0009 46.1 40.9
0.027 17 -- -- -- -- -- -- 0.005 0.0008 49.5 44.1 0.077 18 0.23 --
-- 0.18 0.16 -- 0.003 -- 47.6 41.1 0.059 19 -- 0.18 0.11 -- -- Ce:
0.28 -- 0.0007 47.2 41.8 0.034 20 0.17 -- -- -- 0.38 Nd: 0.17 0.002
0.0009 47.6 41.7 0.047 21 -- -- -- -- -- -- -- -- 48.4 44.3 0.067
22 -- -- -- -- -- -- -- -- 50.8 45.2 0.077 23 -- -- -- -- -- -- --
-- 45.4 41.1 0.044 24 -- -- -- -- -- -- -- -- 41.7* 36.8 0.073 25
-- -- -- -- -- -- -- -- 54.0 47.0* 0.055 26 -- -- -- -- -- -- -- --
54.8 47.7* 0.048 27 -- -- -- -- -- -- -- -- 54.4 47.2* 0.039 28 --
-- -- -- -- -- -- -- 53.4 46.6* 0.077 Numerals marked with "*" mean
that their values are out of the range of the present
invention.
Referring to Table 1, the steels of marks 1 to 20 fell into the
range of the present invention. On the other hand, the chemical
compositions of marks 21 to 28 were out of the range of the present
invention.
The cast piece of each mark was rolled by a blooming mill to
manufacture a round billet. The round billet of each steel had a
diameter of 232 mm. Then, the outer surface of each round billet
was cut such that the diameter of the round billet was 225 mm.
Each round billet was heated to 1150 to 1200.degree. C. in a
reheating furnace. After heating, each round billet was hot rolled.
To be specific, the round billet was piercing-rolled by a piercing
machine to manufacture a hollow shell. The hollow shell was drawn
and rolled by a mandrel mill, and was further reduced in diameter
such that the outer diameter of the hollow shell was 196.9 to 200
mm and the wall thickness was 15 to 40 mm. All the cooling of the
hollow shell after hot rolling was performed by spontaneous
cooling.
Quenching was performed on the hollow shell after it was allowed to
cool. To be specific, the hollow shell was charged into a heat
treatment furnace to be soaked at 980.degree. C. for 20 minutes.
The hollow shell after soaking was water cooled by a spray method
to be quenched. The hollow shell after quenching was soaked at a
tempering temperature of 550.degree. C. for 30 minutes to be
tempered.
Through the above described processes, a plurality of seamless
steel pipes of plural sizes were manufactured at each mark.
The manufactured seamless steel pipes were used to perform the
following evaluation tests.
[Tensile Test]
Round bar specimens (dia. 6.35 mm.times.GL 25.4 mm) conforming to
API specification were taken from a plurality of seamless steel
pipes of each mark. The tensile direction of the round bar specimen
was set to a pipe axis direction of the seamless steel pipe. By
using the prepared round bar specimens, tensile tests were
conducted at normal temperature (25.degree. C.) conforming to API
specification.
After the tensile test, among the plurality of seamless steel pipes
of each mark, the seamless steel pipe having a maximum yield stress
at each mark (hereafter, referred to as a high YS material) and the
seamless steel pipe having a minimum yield stress (hereafter,
referred to as a low YS material) were selected. The high YS
material and the low YS material of each mark were used to perform
the following evaluation test.
[Metal Micro-Structure Observation]
Samples for micro-structure observation were taken from arbitrary
locations of the high YS material and the low YS material of each
mark. In a sample taken, a sample surface of a cross section normal
to the axial direction of the seamless steel pipe was ground. After
grinding, the ground sample surface was etched by using a mixed
solution of aqua regia and glycerin. The area ratio of ferrite
phase on the etched surface was measured by the point counting
method conforming to JIS G0555. The measured area ratio was defined
as the volume ratio of ferrite phase.
Further, the volume ratio of retained austenite phase was
determined by the above described X-ray diffraction method.
Furthermore, based on the determined volume ratios of ferrite phase
and retained austenite phase, the volume ratio of martensite phase
was determined by the above described method.
[Toughness Test]
Full size specimens (L direction) conforming to ASTM E23 were taken
from a high YS material and low YS material of each mark. The
Charpy impact test was performed by using the full size specimen to
determine an absorbed energy at -10.degree. C.
[High-Temperature Corrosion Resistance Test]
Four-point bending test specimens were taken from a high YS
material and low YS material of each mark. The specimen had a
length of 75 mm, a width of 10 mm, and a thickness of 2 mm. Each
specimen was given a deflection by four-point bending. In this
occasion, the deflection amount of each specimen was determined
conforming to ASTM G39 such that the stress given to the specimen
is equal to the yield stress of the specimen.
An autoclave of 200.degree. C. in which CO.sub.2 of 30 bar and
H.sub.2S of 0.01 bar were sealed under pressure was prepared. Each
specimen subjected to a deflection was stored in each autoclave.
Each specimen was immersed in an aqueous solution containing 25 wt
% NaCl+0.41 g/L CH.sub.3COONa (pH=4.5 in CH.sub.3COONa+CH.sub.3COOH
buffer system) in each autoclave for one month.
After 720 h immersion, the occurrence or nonoccurrence of stress
corrosion cracking (SCC) was investigated on each specimen. To be
specific, the cross section of a portion of each specimen to which
tensile stress is applied was observed by an optical microscope
having a visual field of 100 magnifications to determine the
presence or absence of a crack.
Further, the weight of the specimen before and after the test was
measured. A corrosion loss of each specimen was determined based on
the amount of change in the measured weight. From the corrosion
loss, an annual corrosion loss (mm/y) was calculated.
[SSC Resistance Test at Normal Temperature]
Round bar specimens for NACE TM0177 METHOD A were taken from a high
YS material and low YS material of each mark. The sizes of the
specimen were 6.35 mm in diameter and 25.4 mm in GL. A tensile
stress was applied to each specimen in its axial direction. At this
moment, in conformity to NACE TM0177-2005, the deflection amount of
each specimen was determined such that the stress given to each
specimen was 90% of the yield stress (actual measurement) of each
specimen.
The test bath was a 25 wt % aqueous solution of NaCl in which 0.01
bar of H.sub.2S and 0.99 bar of CO.sub.2 were saturated. The pH of
the test bath was regulated to be 4.0 by a
CH.sub.3COONa/CH.sub.3COOH buffer solution containing 0.41 g/L of
CH.sub.3COONa. The temperature of the test bath was 25.degree.
C.
A round bar specimen was immersed in the above described test bath
for 720 hours. After immersion, determination was made on whether
or not cracking (SSC) occurred in each specimen by the same method
as in the high-temperature corrosion resistance test.
[Investigation Results]
Table 2 shows the test results.
TABLE-US-00002 TABLE 2 Low YS Materials High YS Materials Tough-
Corrosion Tough- Corrosion YS F M A ness Resistance YS F M A ness
Resistance Mark (MPa) (vol. %) (vol. %) (vol. %) (J) SCC SSC (MPa)
(vol. %) (vol. %) (vol. %) (J) SCC SSC 1 889 30 68 2 .gtoreq.150 NF
NF 939 29 70 1 .gtoreq.150 NF NF 2 834 43 54 3 .gtoreq.150 NF NF
875 39 56 5 .gtoreq.150 NF NF 3 916 42 56 2 .gtoreq.150 NF NF 974
38 61 1 .gtoreq.150 NF NE 4 868 45 54 1 .gtoreq.150 NF NF 903 36 63
1 .gtoreq.150 NF NF 5 792 33 59 8 .gtoreq.150 NF NF 817 29 69 2
.gtoreq.150 NF NF 6 792 38 55 7 .gtoreq.150 NF NF 813 34 59 7
.gtoreq.150 NF NF 7 779 47 49 4 .gtoreq.150 NF NF 830 44 54 2
.gtoreq.150 NF NF 8 765 44 48 8 .gtoreq.150 NF NF 861 42 54 4
.gtoreq.150 NF NF 9 813 38 56 6 .gtoreq.150 NF NF 868 35 61 4
.gtoreq.150 NF NF 10 799 36 56 8 .gtoreq.150 NF NF 826 37 57 6
.gtoreq.150 NE NF 11 882 35 63 2 .gtoreq.150 NF NF 903 33 66 1
.gtoreq.150 NF NF 12 772 31 60 9 .gtoreq.150 NF NF 896 30 67 3
.gtoreq.150 NF NF 13 765 34 58 8 .gtoreq.150 NF NF 792 29 67 4
.gtoreq.150 NF NF 14 841 45 52 3 .gtoreq.150 NF NF 852 43 55 2
.gtoreq.150 NF NF 15 847 44 52 4 .gtoreq.150 NF NF 898 43 56 1
.gtoreq.150 NF NF 16 882 39 59 2 .gtoreq.150 NF NF 923 37 62 1
.gtoreq.150 NF NF 17 785 38 54 8 .gtoreq.150 NF NF 841 37 58 5
.gtoreq.150 NF NF 18 813 43 52 5 .gtoreq.150 NF NF 863 38 59 3
.gtoreq.150 NF NE 19 889 43 56 1 .gtoreq.150 NF NF 965 35 64 1
.gtoreq.150 NE NE 20 818 36 58 6 .gtoreq.150 NF NF 871 31 67 2
.gtoreq.150 NF NF 21 696 42 46 12 .gtoreq.150 NF NF 813 44 49 7
.gtoreq.150 NF NF 22 723 41 46 13 .gtoreq.150 NF NF 779 42 50 8
.gtoreq.150 NF NE 23 930 39 60 1 86 NF NF 971 36 63 1 83 NF NF 24
841 37 59 4 .gtoreq.150 F F 877 35 63 2 .gtoreq.150 F F 25 668 37
48 15 .gtoreq.150 NF NF 723 39 50 11 .gtoreq.150 NF NF 26 675 40 46
14 .gtoreq.150 NF NF 737 36 52 12 .gtoreq.150 NF NF 27 703 39 49 12
.gtoreq.150 NF NF 777 38 54 8 .gtoreq.150 NF NF 28 682 42 42 16
.gtoreq.150 NF NF 703 37 50 13 .gtoreq.150 NF NF
The "low YS material" column in Table 2 shows evaluation test
results using the low YS material of each mark, and the "high YS
material" column shows the results using the high YS material. "F"
(%) in Table 2 shows the volume ratio (%) of ferrite phase in the
metal micro-structure of a corresponding mark, "M" shows the volume
ratio (%) of martensite phase, and "A" shows the volume ratio (%)
of retained austenite phase, respectively. "NF" in the "SCC" and
"SSC" columns of "Corrosion resistance" column shows that SCC or
SSC was not observed in a corresponding mark. "F" shows that SCC or
SSC was observed in a corresponding mark.
[Regarding Metal Micro-Structure and Yield Strength]
Referring to Table 2, the chemical compositions of the seamless
steel pipes of marks 1 to 20 were within the range of the present
invention and satisfied Formulas (1) and (2), and the metal
micro-structures were also within the range of the present
invention. For that reason, the yield strength of any of the
seamless steel pipes of each mark was not less than 758 MPa (110
ksi) even in low YS, and thus a yield strength of not less than 110
ksi was stably obtained.
Further, there was a tendency observed that a yield strength of a
125 ksi level was obtained even in low YS materials for marks 1, 3,
4, 11, 16, and 19 for which the left hand side value of Formula
(3), that is, the value of F3 was not more than 0.045 among the
seamless steel pipes of marks 1 to 20. Moreover, in marks 5, 6, 8,
10, 12, 13, and 17 in which the value of F3 exceeded 0.060, it was
recognized in low YS materials that although a yield strength of
110 ksi level was satisfied, there was a tendency observed that the
yield strength at the same level of F2 was somewhat lower compared
with the case where the value of F3 was not more than 0.0045 at a
value of F2 of the same level.
Further, in the seamless steel pipes of marks 1 to 20, the
absorption energy at -10.degree. C. was not less than 150 J,
exhibiting high toughness. Further, no SCC was observed at the
high-temperature corrosion resistance test, and also no SSC was
observed in the SSC resistance test at normal temperature.
Note that the corrosion rate was less than 0.10 mm/y in any of
marks 1 to 28.
On the other hand, in marks 21 and 22, the Co content was less than
the lower limit of Co content of the present invention. For that
reason, the yield stress of low YS material became less than 758
MPa, and the volume ratio of retained austenite phase exceeded 10%
as well. Therefore, it was not possible to stably obtain a strength
not less than 110 ksi.
In mark 23, the Co content exceeded the upper limit of Co content
of the present invention. For that reason, both the high YS
material and the low YS material had an adsorption energy at
-10.degree. C. less than 150 J (83 J in the high YS material and 86
J in the low YS material), exhibiting a low toughness.
Although the content of each element of mark 24 was within the
range of the present invention, it did not satisfy Formula (1). For
that reason, SSC was observed in the SSC resistance test,
exhibiting a low SSC resistance. Moreover, SCC was observed in the
high-temperature corrosion resistance test, exhibiting a low
high-temperature corrosion resistance.
Although the content of each element of marks 25 to 28 was within
the range of the present invention, it did not satisfy Formula (2).
For that reason, in all of the low YS materials, the volume ratio
of retained austenite phase exceeded 10%, and the yield strength
was less than 758 MPa (110 ksi). Although there was a case where
the yield strength was not less than 758 MPa as in the high YS
material of mark 27, it was clear that when the value of F2 did not
satisfy Formula (2), a high strength steel pipe could not be stably
manufactured.
Although so far embodiments of the present invention have been
described, the above described embodiments are merely examples for
carrying out the present invention. Therefore, the present
invention will not be limited to the above described embodiments,
and can be carried out by appropriately modifying the above
described embodiments within a range not departing from the spirit
of the invention.
INDUSTRIAL APPLICABILITY
The stainless steel for oil wells according to the present
invention can be utilized in oil wells and gas wells. Particularly,
it can be used in a deep oil well having a high-temperature
environment.
* * * * *